# std::atomic<T>::compare_exchange_weak, std::atomic<T>::compare_exchange_strong

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< cpp‎ | atomic‎ | atomic

bool compare_exchange_weak( T& expected, T desired, std::memory_order success, std::memory_order failure ) noexcept; | (1) | (since C++11)
bool compare_exchange_weak( T& expected, T desired, std::memory_order success, std::memory_order failure ) volatile noexcept; | (2) | (since C++11)
bool compare_exchange_weak( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept; | (3) | (since C++11)
bool compare_exchange_weak( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; | (4) | (since C++11)
bool compare_exchange_strong( T& expected, T desired, std::memory_order success, std::memory_order failure ) noexcept; | (5) | (since C++11)
bool compare_exchange_strong( T& expected, T desired, std::memory_order success, std::memory_order failure ) volatile noexcept; | (6) | (since C++11)
bool compare_exchange_strong( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) noexcept; | (7) | (since C++11)
bool compare_exchange_strong ( T& expected, T desired, std::memory_order order = std::memory_order_seq_cst ) volatile noexcept; | (8) | (since C++11)

Atomically compares the object representation(until C++20)value representation(since C++20) of *this with that of expected. If those are bitwise-equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value stored in *this into expected (performs load operation).

Overloads | Memory model for
--- | ---
read‑modify‑write operation | load operation
(1,2,5,6) | success | failure
(3,4,7,8) | order | std::memory_order_acquire if order is std::memory_order_acq_rel std::memory_order_relaxed if order is std::memory_order_release otherwise order

If failure is stronger than success or(until C++17) is one of std::memory_order_release and std::memory_order_acq_rel, the behavior is undefined.

It is deprecated if std::atomic<T>::is_always_lock_free is false and any volatile overload participates in overload resolution. | (since C++20)

### Parameters

expected | - | reference to the value expected to be found in the atomic object
desired | - | the value to store in the atomic object if it is as expected
success | - | the memory synchronization ordering for the read-modify-write operation if the comparison succeeds
failure | - | the memory synchronization ordering for the load operation if the comparison fails
order | - | the memory synchronization ordering for both operations

### Return value

true if the underlying atomic value was successfully changed, false otherwise.

### Notes

The comparison and copying are bitwise (similar to std::memcmp and std::memcpy); no constructor, assignment operator, or comparison operator are used.

`compare_exchange_weak` is allowed to fail spuriously, that is, acts as if *this != expected even if they are equal. When a compare-and-exchange is in a loop, `compare_exchange_weak` will yield better performance on some platforms.

When `compare_exchange_weak` would require a loop and `compare_exchange_strong` would not, `compare_exchange_strong` is preferable unless the object representation of `T` may include padding bits,(until C++20) trap bits, or offers multiple object representations for the same value (e.g. floating-point NaN). In those cases, `compare_exchange_weak` typically works because it quickly converges on some stable object representation.

For a union with bits that participate in the value representations of some members but not the others, compare-and-exchange might always fail because such padding bits have indeterminate values when they do not participate in the value representation of the active member.

Padding bits that never participate in an object's value representation are ignored. | (since C++20)

### Example

Compare-and-exchange operations are often used as basic building blocks of lock-free data structures.

Run this code

    #include <atomic>

    template<typename T>
    struct node
    {
        T data;
        node* next;
        node(const T& data) : data(data), next(nullptr) {}
    };

    template<typename T>
    class stack
    {
        std::atomic<node<T>*> head;
    public:
        void push(const T& data)
        {
            node<T>* new_node = new node<T>(data);

            // put the current value of head into new_node->next
            new_node->next = head.load(std::memory_order_relaxed);

            // now make new_node the new head, but if the head
            // is no longer what's stored in new_node->next
            // (some other thread must have inserted a node just now)
            // then put that new head into new_node->next and try again
            while (!head.compare_exchange_weak(new_node->next, new_node,
                                               std::memory_order_release,
                                               std::memory_order_relaxed))
                ; // the body of the loop is empty

    // Note: the above use is not thread-safe in at least
    // GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
    // MSVC prior to 2014-03-17 (bug 819819). The following is a workaround:
    //      node<T>* old_head = head.load(std::memory_order_relaxed);
    //      do
    //      {
    //          new_node->next = old_head;
    //      }
    //      while (!head.compare_exchange_weak(old_head, new_node,
    //                                         std::memory_order_release,
    //                                         std::memory_order_relaxed));
        }
    };

    int main()
    {
        stack<int> s;
        s.push(1);
        s.push(2);
        s.push(3);
    }

Demonstrates how `std::compare_exchange_strong` either changes the value of the atomic variable or the variable used for comparison.

| This section is incomplete Reason: more practical use of the strong CAS would be nice, such as where Concurrency in Action uses it

Run this code

    #include <atomic>
    #include <iostream>

    std::atomic<int> ai;

    int tst_val = 4;
    int new_val = 5;
    bool exchanged = false;

    void valsout()
    {
        std::cout << "ai = " << ai
    	      << "  tst_val = " << tst_val
    	      << "  new_val = " << new_val
    	      << "  exchanged = " << std::boolalpha << exchanged
    	      << '\n';
    }

    int main()
    {
        ai = 3;
        valsout();

        // tst_val != ai   ==>  tst_val is modified
        exchanged = ai.compare_exchange_strong(tst_val, new_val);
        valsout();

        // tst_val == ai   ==>  ai is modified
        exchanged = ai.compare_exchange_strong(tst_val, new_val);
        valsout();
    }

Output:

    ai = 3  tst_val = 4  new_val = 5  exchanged = false
    ai = 3  tst_val = 3  new_val = 5  exchanged = false
    ai = 5  tst_val = 3  new_val = 5  exchanged = true

### See also

atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) | atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template)

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